Nerve allograft (a graft between genetically different members of the same species) rejection can be prevented by treating the host with the immunosuppressive drug cyclosporin-A (Cy-A). We have used Cy-A to demonstrate that during immunosuppression host axons will regenerate through a long (4 cm or more) nerve allograft and reinnervate denervated tissue. Further studies were carried out to determine (A) the response of long-term surviving nerve allografts to injury and (B) whether viable cells were needed in the nerve allograft. (A) Our results demonstrated that after injury (crush or cut) of 3-month-old allografts (which contained regenerated host axons) in Cy-A treated rats, the allografts underwent Wallerian degeneration which was followed by the regrowth of host axons. This finding indicated that nerve allografts behave like normal nerves after injury in that they permit repeated axonal regeneration through them. (B) To determine the role of cell viability in nerve allografts, the grafts were frozen prior to their insertion into Cy-A treated rats. We found that, after 3 months, host axonal growth into frozen nerve allografts was restricted to the initial cm of a 4-cm graft. This observation demonstrated that nerve graft matrix alone was not sufficient to permit host nerve fiber regeneration over a long distance. Other data revealed that Cy-A prevented neuronal allograft rejection in the central nervous system (spinal cord) of sensitized rats. In addition, hamster neurons survived in Cy-A treated rats whereas guinea pig neurons were rejected. Finally, histochemical studies of normally myelinated and remyelinated axons revealed a high activity of gamma-glutamyl transpeptidase and Na-ATPase at the paranode. The physiological meaning of this paranodal enzyme localization remains to be determined.